This invention relates to blood pressure measuring methods and apparatus.
During each heart beat, human blood pressure normally rises from about 80 mm of mercury, called diastolic pressure, to about 120 mm of mercury, called systolic pressure. One common method of measuring these pressures, is to inflate a cuff encircling the patient's arm to a pressure above the individual's systolic pressure normally, about 140 mm of mercury, so that the patient's artery is collapsed and no blood flows. The pressure in the cuff is then gradually reduced to a pressure below the patient's systolic pressure, so that the artery opens and blood flow commences. This pressure is noted as the patient's systolic pressure. A contact microphone or other sensor is used to detect the commencement of arterial blood flow which occurs when the patient's blood pressure exceeds that of the cuff. The signal detected by the contact microphone is commonly called the Korotkoff "sound," although the signal power is normally below audible frequencies, that is, about 20 Hz. There is some difference of opinion as to whether the Korotkoff "sound" is generated by the opening of the artery which was previously compressed by the occluding cuff or the actual commencement of blood flow. As the pressure within the artery falls between heart beats, the artery is again occluded. This continues until the cuff pressure falls below the diastolic pressure in which case, the artery remains open. The pressure at the point where the Korotkoff "sound" ceases is the measurement of the patient's diastolic pressure.
One of the problems encountered with prior art blood pressure measuring systems is that in addition to the Korotkoff sound, contact microphones and other sensing devices also detect noise signals which may result from any number of causes, such as muscular contractions and the like. One prior art method of attempting to compensate for noise signals is to employ a pair of sensors one of which is located proximal and the other distal relative to the heart. Each sensor detects the Korotkoff signal plus noise signals. The signals in the two sensors are subtracted which can have the effect of doubling the Korotkoff signal strength relative to random noise and even eliminate common mode noise but has the disadvantage of also passing any portion of the noise which is not of identical magnitude in both sensors.